The present invention is related to a method and apparatus for feeding a chemical into a liquid flow. The method and apparatus of the invention are particularly well applicable to homogeneous adding of a liquid chemical into a liquid flow. Preferably the method and apparatus according to the invention are used for feeding a retention aid into fiber suspension going to the headbox of a paper machine.
Naturally, there is practically an innumerable amount of prior art methods of feeding various chemicals into liquid flows. These methods may be divided into a few main categories, though, as seen from the following. Firstly, it is quite possible to just let the liquid to be added flow freely into a second liquid without employing any special regulation or mixing means. This method of adding can not be employed in situations where the mixing ratio or homogeneity is of significance. Neither can it be employed in situations where the price of the chemical to be added is of significance. The next applicable method is to feed the chemical in a strict proportion to the liquid flow, whereby correct and economical proportioning is obtained. However, even in this case one has to take into account that usually the proportion of the chemical is slightly excessive compared to the optimal proportioning, because the mixing is known to be inadequate. The mixing may be improved, though, by feeding the chemical e.g. through a perforated wall of a flow channel, whereby the chemical to be mixed may at least be spread throughout the liquid flow. Lastly, a situation may be considered, where the chemical is fed in a strict proportion either into the liquid flow on the upper-flow side of the mixer or through the mixer itself into the liquid. In that case, the efficiency of the mixing of the chemical into the liquid flow is totally dependent on the mixer design.
Papermaking is in its own way a very demanding special field when chemical mixing is concerned. When using paper chemicals, it is good to bear in mind that their precise and homogeneous mixing is of vital importance in the short circulation of a paper machine. Homogeneous mixing means in a direct sense better quality and homogeneity of paper. At the same time, the process may be carried out without disturbances and problems. Poor mixing, on the other hand, requires chemical overdosing, which may increase the production costs remarkably. It is self-evident that in case of poor mixing, the quality of the paper and the operation of the process are not satisfactory. The existing mixing technique utilizes, on the one hand, clean water fractions both as dilution waters and as so-called xe2x80x9cwhip-waterxe2x80x9d which is used in order to intensify the mixing. On the other hand, efforts are made to close the water circulations of paper mills, whereby the feeding dosage of clean water into the system should be decreased, and internally clarified fractions or some non-treated direct flow from the process, such as e.g. filtrates, should be used instead. The existing systems for the mixing of chemicals do not allow or allow only to a small extent the use of water fractions of internal processes.
An essential case of mixing relating to paper manufacture is the mixing of a retention aid into fiber suspension flow going to the headbox of a paper machine. In paper manufacture, retention chemicals are used especially in order to improve the retention of fines at the wire part of the paper machine. As retention aid a chemical is used, long molecular chains of which bind together solid matter particles of the pulp and thus prevent the fines from passing, during the web formation stage, together with water through the wire. The retention aid should be mixed into the pulp as homogeneously as possible in order to gain the maximum effect of the chemical and to avoid variation of paper characteristics caused by retention fluctuations. Mixing, on the other hand, means that the liquid is subjected to a turbulent flow, the shearing forces of which break/may break long molecular chains, which naturally weakens the effect of the retention aid. Nevertheless, there are different kinds of retention aids. Sensitive to the effects of a turbulent flow are, e.g., polyacrylic amides, broken molecular chains of which are not known to be restored to their former length after the turbulence has attenuated, but there are also retention aids (e.g. polyethyleneimines), molecular chains of which are restored to their essentially original length shortly after the turbulence has attenuated.
In the short circulation of a paper machine, the feed point of the retention aid depends to a great extent on the retention aid used, the state of the flow from the feed point to the headbox lip, and the pulp used. The introduction of retention aids sensitive to shearing forces usually takes place immediately after a means (that may be a pump, a screen or a centrifugal cleaner) that causes shearing forces and is placed prior to the headbox, the feeding being carried out either into one spot or e.g. into the accept pipe of each pressure screen. It is also possible to use several retention aids of various types at the same time and introduce them into the fiber suspension by stages. The part of retention aids which is resistant to shearing forces may be fed as early as into the high-consistency pulp or prior to the headbox feed pump, and the part of retention aids which is sensitive to shearing forces is usually introduced not until the fiber suspension feed pipe prior to the headbox.
At present, as feeders of retention aids two types of apparatus are mainly used. A simpler apparatus (FIG. 1a) comprises an annular manifold placed around the pulp flow channel in a distance therefrom, connected by a number of feed pipes (at least four feed pipes) with the pulp flow channel so that the retention aid is discharged via said feed pipes in an even flow to the pulp flowing in the channel. A second possibility (FIGS. 1b and 1c) is to take e.g. two feed pipes crosswise through the flow channel and provide the part of the feed pipes which is left inside the flow channel with retention aid feed holes or slots, through which the retention aid flows in an even stream into the pulp, whereby the mixing result is to some extent better. At present, retention aids are fed into the fiber suspension flow under a relatively small pressure difference, whereby the retention aids form their own flow channels or at least a distinct danger exists that they are channeled inside the fiber suspension flow. In other words, in retention aid feeding it is commonly presumed that after the feeding point of the chemical there is a mixing apparatus that mixes the chemicals homogeneously into the fiber suspension. On the other hand, the amount of retention aid that is fed into the fiber suspension is chiefly based on practical knowledge from experience. This means that in practice retention aids are mixed into fiber suspension in an amount big enough to ensure the desired effect. In fact, this means a remarkable overdosing of retention chemicals (sometimes even by tens of percents) due to not homogeneous mixing.
It is characteristic of retention aids and their introduction that the retention aids are delivered to paper mills, in addition to liquid form, also as powders which are used depending on the paper to be made and the material to be used in an amount of about 200-500 g per one paper ton. A retention aid in powder form is mixed into fresh water in a special mixing tank in a proportion of 1 kg of powder to about 200 liters of clean water. This is because retention aids are known to react with, that is to stick onto, all solid matter particles in the flow very quickly, in about a second, which means that the dilution liquid has to be as clean as possible. In other words, in this stage, per 1 ton of produced paper 40-100 liters of clean water is used for retention aid production. Consequently, the consumption per day is, depending on the production of the paper machine, 10-100 cubic meters (here the production is estimated to be 250-1000 tons of paper per day). Nevertheless, this first dissolution stage is not the stage where water is used at the most, as in prior art processes this retention aid solution is further diluted into, e.g., one fifth of its concentration, which in practice means that for this so-called secondary dilution 200-500 liters of clean water is used per 1 paper ton. This results in a calculated daily consumption of 50-500 cubic meters of clean water per one paper machine.
In other words, until now it has been accepted that for the dilution of the retention aid per one paper machine hundreds of cubic meters of clean water is needed per day. Nevertheless, this has to be understood as a clear drawback, especially in cases when the paper mill is known to have great amounts of various circulation waters available, which might be utilized for this purpose, too. The only precondition for the use of circulation waters is that there should be a way to prevent retention chemicals from reacting with the solid matter in the circulation waters.
On the one hand, one has to bear in mind that the short circulation of a paper machine employs, due to large amounts of liquid, large-sized pipes. For example, as a feed pipe of the headbox of a paper machine, a pipe with a diameter of about 1000 mm may be used. This is one of the reasons why mixing a relatively small additional flow, such as a diluted retention aid, homogeneously into a wide flow channel is problematic.
On the other hand, the construction of the above described, presently used retention aid feeding apparatuses is very simple. When considering their operational efficiency, i.e. the homogeneity of the mixing, one might even say that they are too simple. In other words, the simplicity of the apparatus and the feeding method of chemicals, resulting in non-homogeneous dosing and also degradation of chemical molecules, inevitably lead to remarkable overdosing of chemicals, as the basic goal inevitably is to achieve a certain wire retention on a paper machine.
A further evident problem discovered in prior art processes is connected with the most traditional way of mixing the retention aid into the fiber suspension, that is prior to the headbox screen. Because the reaction time of a retention aid was known to be short, the headbox screen was considered a magnificent place for homogeneous and quick mixing of the retention aid into the pulp. And so it was when headbox screens of old art where used, which had a hole drum as a screening member. But now, with slot drums conquering the market, it has been discovered that the retention aid is capable of forming flocks prior to the slot drum, and thus a great amount of both the retention aid and the fines of the fiber suspension otherwise usable is, at best, rejected or, at worst, clogs the fine slots of the slot drum.
As noticed from above, numerous drawbacks and disadvantages have been discovered for example in the feed of retention chemicals. For solving e.g. the above mentioned problems of prior art, a new method and apparatus have been developed, which allow feeding into the liquid flow even chemicals consisting easily degrading polymeric chains, for instance retention chemicals, so that the polymeric chains remain non-degraded to a remarkably larger extent than before. As another advantage of the method and apparatus according to the invention we may mention, e.g., a substantial decrease in the consumption of fresh water in a paper mill, when desired, and an essentially more efficient and homogeneous mixing of retention aids into the fiber suspension.
According to one aspect of the invention there is provided a method of mixing a first liquid chemical into a second liquid using a mixing apparatus having a mixed-liquid discharge, comprising: (a) Introducing the second liquid into the mixing apparatus so that a second liquid flow is formed. And (b) introducing the first liquid chemical into the mixing apparatus so that the first liquid chemical is substantially simultaneously mixed with the second liquid with the discharge of the chemical and second liquid from the mixing apparatus into a fourth liquid.
According to another aspect of the invention there is provided a method of mixing a first liquid chemical into a second liquid substantially free of solid matter, comprising: (a) Feeding the first liquid chemical into the mixing apparatus so that a spiral flow of the liquid chemical is established. (b) Introducing the second liquid into the mixing apparatus into communication with the spiral flow of liquid chemical. And (c) discharging the second liquid mixed with the liquid chemical, from the mixing apparatus into a fourth liquid.
According to another aspect of the invention there is provided mixing apparatus for mixing a liquid chemical and a second liquid comprising: A casing with inlet conduits therein for the chemical to be mixed and the second liquid and one outlet conduit. A member located inside the casing essentially concentrically with the casing, the member having an outer shell which defines inside the casing an annular space outside the shell and a space inside the shell. And a chemical conduit connected to the space inside the shell.
According to another aspect of the invention there is provided Mixing apparatus for mixing a liquid chemical and a second liquid comprising: A casing having an inlet conduit for the liquid chemical, an inlet conduit for the second liquid, an open interior and a single outlet conduit. And the inlet conduit for the liquid chemical connected to and opening into the casing interior so that chemical ted into the liquid chemical inlet conduit flows spirally within the casing.